Wayne, PA Chanticleer Garden (PA)

Kennett Square, PA Longwood Gardens

Philadelphia Pennsylvania Horticultural Society

Philadelphia Morris Arboretum

Wilmington, Delaware Delaware Horticultural Center

New York City The Horticultural Society of New York

Bronx, NY Wave Hill

Bronx, NY New York Botanical Garden

Boston, MA Massachusetts Horticultural Society

Chicago, IL Chicago Botanic Garden

Florida Florida State Horticultural Society

California The California Horticultural Society

Canada Royal Botanical Gardens 

Canada Montreal Botanic Garden

http://www.rhs.org.uk/ The Royal Horticultural Society

http://www.harlandhandgarden.com/ The Harland Hand Garden (California)

http://www.hr.duke.edu/dukegardens/ The Sarah P. Duke Gardens

http://www.holdenarb.org/ The Holden Arboretum

www.cthort.org/  The Conn Horticultural Society

www.northerngardener.org/  Minnesota State Horticultural Society

www.mihortsociety.org/  Michigan State Horticultural Society

American Hosta Society

N. American Rock Garden Society

http://www.tropicalgardening.com/

National Gardening Association

I Can Garden (Canada)

American Association of Botanical Gardens and Arboreta Umbrella organization with listings for all member organizations.